Polyphosphazenes are a class of polymers comprising a chain of ##STR1## units in which n can range from 3 to 50,000 or higher. The properties of such polymers depends to a large extent on the nature of substituents on the phosphorus atom. One useful class of substituents are aryloxides. Such compositions are described in U.S. Pat. No. 3,853,794; U.S. Pat. No. 3,856,713 and U.S. 3,883,451. Another useful class of substituents comprises alkoxides, both substituted and unsubstituted. These polymers are also known in the art. The prior art makes substituted polphosphazenes by adding a solution of an alkali metal aryloxide or alkoxide generally in an ether solvent such as tetrahydrofuran to a solution of a phosphonitrilic chloride polymer in a different solvent such as benzene, toluene, xylene and the like. These prior art processes entail the use of two different solvents which require separate storage and handling facilities and the solvents cannot be used again in the same process until they are separated and purified by rectification. Thus, there is a major economic incentive to discover a substitution reaction which can be conducted using a single solvent system.
One single solvent process is described in U.S. Pat. No. 4,129,529 which represents a substantial advance in the polyphosphazene art. The requirements of the solvent are that all of the following must be soluble in the single solvent--(1) the phosphonitrilic halide polymer, (2) the alkali metal aryloxide or alkoxide and (3) the substituted polyphosphazene product. These criteria are disclosed to be satisfied by tetrahydrofuran.
Austin et al Macromolecule 16, p. 719-22 (1983) disclose the substitution of phosphonitrilic chloride polymers in a single solvent in which an alkali metal alkoxide or aryloxide would not normally be soluble. Austin et al circumvent this problem by adding a tetra-n-butyl ammonium chloride to the reaction mixture which converts the alkali metal alkoxide or aryloxide to a soluble tetra-n-butyl ammonium salt which migrates to the organic phase. Hence in the Austin et al system the alkoxides and aryloxides are not insoluble in the organic phase.